Preheat oven to 200C.Roll beef filet in herbs and press in well.
Heat large heavy based pan on stove and once very hot add a
little butter and swirl. Add beef and seal on all sides, crusting
well. Transfer to oven and roast for 25 minutes for rare beef, 35
minutes for medium rare. Sit covered for 10 minutes then slice
into 2cm thick medallions and fan onto four plates .

Demi glace can be substituted by 200 ml of water and 1 1/2 stock
cubes. Reduce wine and demi glace by half, add freshly ground
black pepper and off heat, whisk in butter in small pieces until
a smooth and glossy sauce is achieved. After this stage the sauce
must not be boiled again or it will curdle. Pour down the centre
of beef and allow to pool on plates.
Peel the sweet potato and cut into matchsticks, pat dry then fry
in very hot oil. Pat dry and arrange beside beef. Serve
accompanied by green vegetables of your choice.